The present invention relates to a light source device comprising a bulb, a discharge medium mainly composed of a rare gas sealed inside the bulb, and an electrode for exciting the discharge medium. The present invention also relates to a lighting device, such as a back light device, comprising this light source device, and a liquid crystal device comprising this back light device.
Recently, research on a light source device that does not use mercury (hereafter referred to as mercury-less type) as a lamp or light source device used for a back light device of a liquid crystal display device is actively progressing, in addition to research on a light source device using mercury for such usage. The mercury-less type light source device is preferable due to low fluctuation of light emission intensity along with time variation of temperature and in view of consideration of environments.
A known mercury-less light source device has a tubular bulb in which a rare gas is sealed, an internal electrode disposed inside the bulb, and an external electrode disposed outside the bulb. Application of a voltage between the internal electrode and external electrode causes a dielectric barrier discharge, resulting in that the rare gas is converted into plasma to emit light.
Various types of external electrodes are known. For example, a conventional light source device shown in FIG. 31A has a bulb 3 in which a rare gas is sealed and a internal electrode 1 is disposed, and a linear external electrode 2 extending parallel to a central axis or an axis line L of the bulb 3 and disposed so as to closely contact an outer surface of the bulb 3. The external electrode 2 is formed by applying metal paste onto the outer surface of the bulb 3, for example. The internal electrode 1 is electrically connected to a lighting circuit 4, and the external electrode 2 is grounded (for example, see Japanese Patent Application Laid-Open Publication No. 5-29085).
An external electrode, in which a conductive element is mechanically pressed to an outer surface of a bulb, is also known. For example, one of conventional light source devices has an external electrode made of a conductive wire member and wound spirally around a bulb so as to closely contact an outer surface of the bulb (for example, see Japanese Patent Application Laid-Open Publication No. 10-112290). Further, another one of conventional light source devices has an external electrode made of a conductive wire member and wound in a coil manner around an outer surface of a bulb, and a shrink tube that secures the external electrode so as to be closely in contact with the outer surface of the bulb (for example, see Japanese Patent Application Laid-Open Publication No. 2001-325919).
Even if the external electrode 2 is formed by coating with metal paste, the external electrode 2 cannot be completely contacted with the outer surface of the bulb 3. In other words, as shown in FIG. 31B, due to various causes, such as manufacturing error, vibration during operation, and a temperature status of an environment, a void or a slight gap 5 is generated between the external electrode 2 and the bulb 3. If the gap 5 exists, electric power cannot be supplied normally to the bulb 3. This causes instability in light emission intensity. Further, a dielectric breakdown of an atmospheric gas tends to occur at the gap 5, and gas molecules ionized by the dielectric breakdown cause damage to peripheral elements or members. For example, if the atmospheric gas is air, the dielectric breakdown generates ozone that causes damage to the peripheral members.
Even if mechanically pressed onto the outer surface of the bulb, this conductive element is detached from the outer surface of the bulb by deflection of the conductive element. Even if such a measure as a shrink tube is used, it is impossible to completely contact the conductive member with the outer surface of the bulb. Therefore, the above-mentioned gap exists between the external electrode and the outer surface of the bulb without exception, thereby causing unstable light emission and dielectric breakdown of the atmospheric gas.
As discussed above, even in a case of an external electrode formed by a chemical method employing metal paste, deposition, sputtering and adhesive, rather than such a physical method as mechanical pressing and use of a shrink tube, a gap between the external electrode and the outer surface of the bulb inevitably exists. The gap causes unstable emission and dielectric breakdown of the atmospheric gas.